1. Field of the Invention
This invention relates to integrated optoelectronic devices, especially optically connected optical waveguides; and to their method of use for phase modulation, and switching.
2. Description of the Prior Art
The use of optoelectronic devices in integrated optical circuits, instead of integrated electronic devices, or integrated together with electronic devices on one device, is becoming increasingly important. Examples of materials suitable for use in optoelectronic devices include lithium niobate, and silicon. Silicon, as a basis for integrated optoelectronic circuits, is advantageous since it provides the potential use of standard silicon integrated electronic circuit manufacturing technology, and the integration of optical and electronic circuits on one silicon device. Examples of silicon based optoelectronic devices can be found in EP-A-0720754 and other references given therein. Typically optoelectronic integrated circuits comprise optical waveguides formed on a substrate. For example, a device may comprise a layer of silicon separated from a substrate by a layer of insulating material, such as silicon dioxide, with ribs formed in the upper surface of the silicon to provide one or more waveguides.
Information is coded in optoelectronic devices as a variation of some aspect of the optical output from a light source such as an LED (Light Emitting Diode) or a laser diode. The aspect of the optical output, which is typically varied, is the amplitude, the time length of the pulses, or the phase of the optical output. The variation in the optical output may be achieved by direct variation of the output of the light source, or more usually by passing the light through a material whose optical properties (usually the refractive index) can be modified by an external means. For example, the refractive index of certain materials can be modified by applying an electric field (electro-optic effect), applying a magnetic field (magneto-optic effect), or applying heat (thermo-optic effect). The refractive index of certain semiconductor materials, such as silicon, can also be modified by controlling the density of free charge carriers in the material, by so-called "free carrier injection" or "free carrier depletion". Optoelectronic devices which modify the refractive index of their constituent materials by altering the density of free charge carriers are described, for example, in EP-A-0720754. Lithium niobate devices, and the silicon on insulator devices, described above, are typical materials which will exhibit electro-optic, magneto-optic, or thermo-optic behaviour, and whose refractive indices can be altered by carrier injection.
One known example of an optoelectronic device that can be incorporated in an integrated optoelectronic circuit is an interferometer, such as a Mach Zehnder interferometer. Such a device typically comprises a branched silicon waveguide: a stem waveguide dividing into two branches, which recombine to provide an exit waveguide. The branches of the waveguides generally extend substantially parallel to each other over the majority of the length of the interferometer. It is known to use a Mach Zehnder device for switching. This is achieved by introducing a relative phase shift in the light passing through the branches of the waveguide, the exit waveguide of the device registering the switch as "on" when there is no relative phase shift between the two branches of the waveguide, and "off" when there is a relative phase shift of n.times..pi. (where n is an integer) between the two branches of the waveguide. The relative phase shift is introduced by modifying the refractive index in one of the branches of the device. In general, as the refractive index of a transparent material is changed, the phase of light passing through it is changed as well: the bigger the change in refractive index, the bigger the phase change. In one known operation of a Mach Zehnder device, the refractive index of one branch is modified by heating one branch of the waveguide (i.e. utilising the above described thermo-optic effect). For a silicon waveguide, for example, the refractive index increases by 2.times.10.sup.-4 for every degree Centigrade rise in temperature. Switching the device off and on again is achieved by heating one branch of the waveguide (so there is a phase shift between the branches of the waveguide), and then allowing it to cool to the same temperature as the other branch of the waveguide again (so there is no phase shift between the branches of the waveguide). The speed of switching on is determined by how quickly the heat can be applied, and the speed of switching off is determined by how quickly the waveguides reach the same temperature again, i.e. how long the heat takes to flow away from the hotter branch. For most materials, the phase shift achieved is approximately linear relative to the temperature difference between the branches of the waveguide, and the size of the maximum phase shift does not affect the optical losses. The known use of the thermo-optic effect to modify the refractive index, and consequently the phase in a Mach Zehnder interferometer is, however, relatively slow. This is because of the time needed for the heated branch to heat and cool again. Therefore using the known operation of a thermally driven Mach Zehnder device, typically switching is limited to the kHz operating range, i.e. of the order of 10.sup.3 times per second.
It is also known to use carrier injection as a means to modify the refractive index, and consequently the phase, in an interferometer such as a Mach Zehnder device. Careful design of the carrier injection means refractive index changes may be made much more quickly than using the thermo-optic effect. Typically switching using carrier injection in an interferometer can operate in the GHz range, i.e. of the order of 10.sup.9 times per second. Carrier injection usually decreases the refractive index of the material to which it is injected. However carrier injection also tends to heat the material into which it is injected, which tends to increase the refractive index of the material. Thus the two effects act in opposite directions on the refractive index of the material, which disadvantageously limits the maximum phase shift obtainable, and causes the phase shift response to be non-linear relative to the amount of carrier injected and the switching frequency. Also, disadvantageously, carrier induced optical losses may also occur.
We have discovered a method of using two optically connected optical waveguides, and a device incorporating such optical waveguides, which use the thermo-optic effect to introduce a relative phase change between the waveguides, but which have a much faster response than the interferometric devices using the thermo-optic effect known hitherto.